This invention relates to ladle heating methods and apparatus. More particularly, this invention pertains to rapid high temperature ladle preheating utilizing an optimized heating cycle by involving oxygen and combustion air preheated by recuperation in the fuel burning process. For the purpose of this application, the word "ladle" shall be understood to include tundishes, AOD vessels, BOF vessels, and other refractory vessels for holding molten metal.
Refractory lined ladles used in molten metal handling are normally preheated prior to use and also are kept hot during idling time when they are waiting for a metal charge. Also, ladles must be heated when drying new or repaired refractory linings.
Ladle preheating stations used for firing into the ladle interior during preheating, reheating and drying have been very inefficient because of primitive combustion controls and high energy losses. Three major components of energy loss occur during ladle heating: (1) heat is lost with the flue gases due to the high temperature required and inadequate fuel/air ratio control, (2) heat is lost due to cold air infiltration through the gap between the ladle rim and the refractory lined wall of ladle preheating stations, and (3) heat is lost by radiation from the flame, the ladle and the heater wall refractory lining through said gap between the ladle and the heater.
Some prior art patents have attempted to overcome these energy losses. For instance, techniques of heat recovery during ladle preheating by recuperation are disclosed in U.S. Pat. Nos. 1,057,905 and 4,229,211. Here, different methods of sealing of the ladle rim are applied to direct flue gases into a recuperator for preheating of combustion air. Both of these methods of sealing create inconveniences in actual operation. The method disclosed in U.S. Pat. No. 1,057,905 requires the ladle to be inverted which is an unacceptable posture for large steel mill ladles. The method of U.S. Pat. No. 4,229,211 cannot be properly utilized in many cases without clearing the rim of the ladle to remove solidified metal and slag, and the heater wall must be relined frequently due to damage to the compressible lining of the seal assembly of the heater by docking with the rough rim of the ladle.
The method disclosed in U.S. Pat. No. 4,359,209 tries to overcome these problems by arranging an air seal around entire gap between the rim of the ladle and the wall of the preheating station by discharging flue gases through said gap and then, after mixing said flue gases with ambient air outside of the ladle interior, directing this mixture into the recuperator to preheat the combustion air. The disadvantage of this method and apparatus is that in practice the dimension of said gap has to be at least equal to the thickness of local deposition of solidified metal at the ladle spout, which is typically between 4" and 12". Such a gap cannot be overcome with the amount of hot combustion gases that are typically generated by the burner in the ladle interior. Another disadvantage of this system is that radiative heat losses through the gap still occur.
It is also desirable to reduce the time for completion of the ladle heater heating cycle.
The heating of a ladle consists of two stages: (1) preheating to the desired temperature of the hot face of the lining, and (2) soaking heat into the refractory lining by keeping the hot face at the desired temperature. Initially, the heat flux introduced into the ladle refractory lining throughout its hot face is stored in the refractory material located close to the hot surface of the lining, and only a relatively small part of the heat is transferred throughout the refractory lining due to the low thermal diffusivity of the ladle refractory's lining materials. This results in a sharp reduction of the refractory's ability to accept heat as the ladle heating cycle proceeds. New advanced refractory materials such as dolomite and magnesite are used today for high quality steel products. Compared to conventional refractories, these materials require 50% more heat storage in the ladle refractory lining (about 30,000 Btu/ft.sup.2) and a temperature of 2100.degree. F. or higher for the hot face of the ladle refractory. At the same time, the significantly higher price of said refractories have forced steel makers to minimize the number of ladles they have in operation, which reduces the preheating cycle time. Therefore, increased intensity of ladle heating has become desirable in addition to increased energy recovery efficiency. The shortening of the heating cycle, together with the increase of heat storage required in the refractory lining, demands a higher heat flux from the flame to the ladle lining being heated.
Efficient energy recovery under such conditions requires a very complicated recuperator containing radiant and convective heat exchangers having impractically high turn down ratios. Such a method of heat recovery is especially inappropriate for ladle heating due to the necessity to maintain a very high pressure drop through the recuperator and, therefore, a nearly perfect seal between the rim of the ladle and the ladle heating station.
Because of this, existing recuperative ladle heaters are designed to meet the conditions of the soaking cycle. But this is when the flue gas temperature is at a maximum and the air flow is significantly reduced, so the danger of overheating the recuperator has to be addressed. Therefore, during the preheating cycle, the ability to increase the heat input from conventional ladle heating stations is limited and results in the loss of heat recovery efficiency and increases the duration of the preheating cycle.